@Assault Pioneer: PDW aka personal defense weapon aka home defense weapon. It's not aimed at MIL/LE as they have much better options, but at civilians more than anything
@Tarobear: To mount optics/grips, get better control of the recoil, etc. There's not that many options for a .45 PDW on the civilian market.
http://www.hera-arms.com/cpe.php
In RS, you can also intall longer barrels that would be totally unusable on the pisol alone (shitty handling). Again, for a PDW it makes a lot of sense.
YouTube - Hera-Arms CPE Colt 1911A1 Carbine Conversion
Make that kind of follow-up shots on a RS .45 pistol while keeping good precision... I don't think it's easy without looooots of training.
Again, that's for the RS... but some of it still applies to airsoft.